1. Field of the Invention
The present invention is generally related to nuclear fuel elements and in particular to particle bed fuel elements.
2. General Background
Particle Bed Reactors (PBR's) first conceived in the early sixties were intended to be an improvement over the solid core reactors developed in conjunction with rocket propulsion programs. The PBR's then proposed consisted generally of an annular bed of nuclear fuel particles held on the outside by a porous tube (outer frit). In some concepts the particle bed was held against the outer frit by centrifugal force by rotating the bed and frit. This was referred to as the rotating bed. In another concept referred to as the fixed bed the particles were held between an inner and outer frit. In both concepts coolant gas entered through the outer frit, flowed radially inward through the fuel particle bed where it was heated, and exhausted axially along the central void in the bed. Another variation of the particle bed reactor involved an array of discrete fuel elements consisting of an outer and inner frit, a bed of fuel particles and suitable end fittings to hold the assembly together. These fuel elements typically have a length to outside diameter ratio of four to one or larger whereas the initial rotating bed and fixed bed concepts generally had a ratio of about one to one. During operation nuclear fuel elements present problems associated with growth of fuel particles during burnup and differential thermal expension and contraction of the particle bed and inner and outer frits. During changes in component temperatures differential thermal expansions or contractions may cause the fuel particle bed to undergo increased compression or, conversely, compressive loads may be removed, causing the bed to become loose. Excessive compressive loads could cause the fuel particles to fail by fracturing or the confining frits to distort. A loose bed may allow coolant gas to flow more freely through some areas than others, resulting in undesirable cold and hot spots within the fuel particle bed. A properly designed particle bed reactor fuel element must be able to accommodate specified changes in temperature and resulting differential expansions.
One known approach to the above problem has been to provide a wave spring positioned at each end of the fuel bed.
Patents known to the applicant which tend to address the differential expansion/contraction problem include the following.
U.S. Pat. No. 3,679,545 to Leirvik discloses a nuclear reactor core element utilizing one or more corrugated tubular spacer members that resiliently support and locate within a sealed cladding tube either nuclear fuel material or neutron absorber material. Each corrugated spacer is located in a plenum zone within the cladding tube and radially reinforces the tube wall section around such plenum zone against collapse by external fluid pressure.
U.S. Pat. No. 3,989,590 to Wehrli, III et al. discloses an internally pressurized hermetically clad fuel element for a nuclear reactor having a sealed collapsible capsule within the fuel element plenum. The capsule is controllably collapsed in a buckling mode with increases in plenum pressure to maintain the fuel element pressure substantially constant throughout its operating life.
U.S. Pat. No. 3,647,623 to Hepps et al. discloses a metallic clad fuel element for nuclear reactors which has a bellows-like member internally supported therein and communicably coupled with the environment of the fuel element so as to maintain an internal pressure substantially equal to the external or environmental pressure during burnup.
U.S. Pat. No. 3,677,894 to Ferrari discloses a fuel element for a pressurized nuclear reactor comprising a sealed cladding case, nuclear fuel therein and means for producing an internally pressurized atmosphere so that the fuel element is free standing, and characterized by metal wall cladding having a reduced thickness for conditions of reactor use.
U.S. Pat. Nos. 3,009,869 to Bassett, 3,772,147 to Bratton et al., 3,274,067 to Greebler et al., 3,291,698 to Fortescue, 3,671,393 to Williams, 4,011,134 to Stehle et al., 4,699,757 to Cloue, and 4,111,748 to Hayashi et al. are directed toward a variety of spacer members representative of the known state of the art.
The patents are all directed toward sealed fuel elements having plenum chambers to trap and retain fission gases produced by the fuel therein during burnup while also providing for some longitudinal expansion of fuel segments. This is different from the problem addressed by the present invention, differential expansion and contraction of porous, not sealed, fuel elements and fuel particles wherein the cooling gas flows directly through a bed of fuel particles, not large fuel segments, in direct contact with the fuel particles. The only approach to this problem as mentioned above is considered insufficient because of its limitation on the length-to-width ratio of the fuel element. A solution to the problem is needed wherein expansion and contraction may occur without causing damage to or looseness of the fuel particles, a significant amount of fuel is not displaced, a significant amount of neutron absorbing material is not added, and there is no significant interference with the proper distribution of coolant gas flow.